<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    .bg {
      background-color: yellow;
    }
  </style>
</head>
<body>
    <input type="text" id="txtUserName"> <br>
    <input type="password" id="txtUserPassword"> <br>
    <input type="button" value=" 登 录 " id="btnLogin">
    <script>
      // 检测用户名是否是3-6位，密码是否是6-8位，如果不满足要求高亮显示文本框
      var btnLogin = document.getElementById('btnLogin');

      btnLogin.onclick = function () {
        // 检测用户名是否是3-6位，密码是否是6-8位
        var txtUserName = document.getElementById('txtUserName');
        var txtUserPassword = document.getElementById('txtUserPassword');

        //检测用户名是否是3-6位
        if (txtUserName.value.length < 3 || txtUserName.value.length > 6) {
          txtUserName.className = 'bg';
          return;
        } else {
          txtUserName.className = '';
        }

        // 密码是否是6-8位
        if (txtUserPassword.value.length < 6 || txtUserPassword.value.length > 8) {
          txtUserPassword.className = 'bg';
          return;
        } else {
          txtUserPassword.className = '';
        }

        // 
        console.log('执行登录');
      }
    </script>
</body>
</html>
